1. Field of the Invention
The present invention relates to a system and a method for handover in a mobile communication system, and more particularly to a system and a method for optimizing a handover process, which can reduce handover time delay in a broadband mobile communication system.
2. Description of the Related Art
In a 4th generation (hereinafter, referred to as a 4G) communication system, which is the next generation communication system, research has been actively pursued to provide users with services having various quality of services (hereinafter, referred to as a QoSs) and supporting a transmission speed of about 100 Mbps. Especially, in a current 4G communication system, research has been actively pursued to develop a new type of communication system ensuring mobility and QoS in the wireless Local Area Network (LAN) system and the wireless Metropolitan Area Network (MAN) system supporting relatively high transmission speeds, and support a high speed service to be provided by the 4G communication system. As representatives of such new type communication systems, an Institute of Electrical and Electronics Engineers (IEEE) 802.16a communication system and an IEEE 802.16e communication system have been developed.
The IEEE 802.16a communication system and an IEEE 802.16e communication system employ an Orthogonal Frequency Division Multiplexing (OFDM) scheme and an Orthogonal Frequency Division Multiple Access (OFDMA) scheme in order to support broadband transmission network for a physical channel of the wireless MAN system.
The IEEE 802.16a communication system is a system for only a single structure without reflecting the mobility of a Subscriber Station (SS) at all, in which the SS is in a stationary state. In contrast, the IEEE 802.16e communication system is a system reflecting the mobility of SS, and the SS having the mobility will be referred to as a Mobile Subscriber Station (MSS).
Hereinafter, the structure of the conventional IEEE 802.16e communication system will be described with reference to FIG. 1 which is a block diagram schematically illustrating a structure of a conventional mobile communication system (e.g. the IEEE 802.16e communication system).
The IEEE 802.16e communication system has a multi-cell structure including a cell 100 and a cell 150. Further, the IEEE 802.16e communication system includes a Base Station (BS) 110 controlling the cell 100, a BS 140 controlling the cell 150, and a plurality of MSSs 111, 113, 130, 151 and 153. The transmission/reception of signals between the BS 110 and 140 and the MSSs 111, 113, 130, 151 and 153 is accomplished using an OFDM/OFDMA method. Herein, the MSS 130 is located in an overlapping area (i.e., a handover area) between the cell 100 and the cell 150. Accordingly, when the MSS 130 moves further into the cell 150 controlled by the BS 140 while transmitting/receiving a signal with the BS 110, a serving BS for the MSS 130 changes from the BS 110 to the BS 140.
Hereinafter, a typical process of handover will be discussed with reference to FIG. 2 which is a flow diagram illustrating a typical handover process.
First, an MSS 210 linked to a serving BS (BS #1) 220 tries to connect to a new BS when a status of the link to the serving BS 220 degrades below a predetermined threshold. In order to determine suitable base stations which can serve as a new serving base station, the MSS 210 scans linkable neighbor BSs, for example, BS #2 230, BS #3 240 and BS #4 250 (Step 201). Thereafter, based on the determination, the MSS 210 selects BSs (e.g., BS #2 230, BS #3 240) to which the MSS 210 will request handover. Then, the MSS 210 transmits to the serving BS 220 an MSS handover request (MOB_MSSHO_REQ) message including the recommended neighbor BSs 230 and 240 and the scanned result (Step 203). Upon receiving the MOB_MSSHO_REQ message, the serving BS 220 transmits a handover pre-notification (HO_pre_notification) message reporting the handover request by the MSS 210 to the recommended neighbor BSs (i.e., BS #2 230 and BS #3 240) contained in the MOB_MSSHO_REQ message (Steps 205 and 207, respectively). The HO_pre_notification message includes a required bandwidth and required service quality information of the MSS 210. Upon receiving the HO_pre_notification message, the neighbor BSs 230 and 240 transmit to the serving BS 220 a response (HO_pre_notification_response) message including approval or disapproval of the handover request of the MSS 210 which is based on their own resource statuses (Steps 209 and 211, respectively). Thereafter, the serving BS 220 determines BSs to which the MSS 210 can be handovered and transmits a handover response (MOB_BSHO_RSP) message including the determined BSs to the MSS 210 (Step 215). Here, the MOB_BSHO_RSP message includes one or more BSs. Thereafter, the serving BS 220 transmits a handover confirmation (HO_confirm) message to the target BS one of the recommended target BSs (e.g. BS #3 240) included in the MOB_BSHO_RSP message so that the BS can prepare the handover of the MSS 210 (Step 213).
Upon receiving the MOB_BSHO_RSP message, the MSS 210 determines a target BS to which handover will be performed from among the recommended target BSs included in the MSS_BSHO_RSP message and transmits to the serving BS 220 a handover indication (MOB_HO_IND) message including the determined BS (Step 217). Upon receiving the MOB_HO_IND message from the MSS 210, the serving BS 220 releases the wireless connection channel with the MSS 210 (Step 219).
Then, the MSS 210 performs a network reentry process with the neighbor BS (e.g. BS #3 240) included in the MOB_HO_IND message (Steps 221-225). Hereinafter, the neighbor BS to which the MSS 210 moves and newly connects will be referred to as a target BS. After performing the network reentry process, the MSS 210 sets a new wireless connection channel with the target BS 240. The network reentry process with the target BS 240 is similar to the network entry process with the serving BS 220 and includes the steps of: (1) downlink/uplink parameter acquisition; (2) ranging; (3) basic capability re-negotiation; (4) re-authorization; (5) re-register; and (6) re-establishment of IP connectivity. Finally, the MSS 210 having established a new wireless connection channel with the target BS 240 can continue the data transmission (Step 227). According to the construction as described above, in the course of releasing the wireless connection channel with the existing serving BS and establishing a new wireless connection channel with the target BS, the existing serving BS deletes the service context of the MSS at the following time point. That is to say, the serving BS deletes the service context at the time point when receiving the MOB_HO_IND message from the MSS or at the time point when receiving a handover completion message from the target BS after establishment of a new wireless connection channel between the MSS and the target BS is completed. Here, the handover completion message has yet to be defined in detail.
The existing handover method in which the MSS releases the existing wireless connection channel with the serving BS and establishes a new wireless connection channel with the target BS at the time of handover as described above has the following disadvantages.
Specifically, according to the existing handover method, frequent handover is necessary when an MSS frequently moves between different cells controlled by different BSs and a network reentry process causing a delay is necessary whenever the handover is performed. Therefore, according to the existing handover method, interruption of data transmission/reception due to handover may increase and become unacceptable.
Further, if the handover process is initiated when the signal from the serving BS has a Carrier to Interference and Noise Ratio (CINR) is smaller than that of a neighbor BS, the MOB_MSSHO_REQ message, the MOB_BSHO_RSP message and MOB_HO_IND message must be transmitted even in a state in which the connection between the MSS and the serving BS is poor. In this case, a transmission error or a delay due to a retransmission may occur which can cause further delays and service interruptions. Moreover, a random access delay may occur when the MSS transmits the MOB_MSSHO_REQ message and the MOB_HO_IND message. Therefore, depending on the length of the time delay, the connection with the serving BS may be interrupted even before the handover process is completed.